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Date: Fri, 4 Feb 2011 13:30:45 +0100
From: Guillaume Yziquel <guillaume.yziquel@citycable.ch>
To: david.baelde@ens-lyon.org
Cc: Nicolas Ojeda Bar <nojb@math.harvard.edu>, caml-list@yquem.inria.fr
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Subject: Re: [Caml-list] reference cells

Le Friday 04 Feb 2011 à 12:49:28 (+0100), David Baelde a écrit :
> On Mon, Jan 31, 2011 at 5:46 PM, Guillaume Yziquel
> <guillaume.yziquel@citycable.ch> wrote:
> >> # let x = ref 10;;
> >> val x : int ref = {contents = 10}
> >> # Obj.tag (Obj.repr x);;
> >> - : int = 0
> 
> A funny side note about that test: the fact that you pass your
> reference to a function (Obj.repr) prevents the compiler from
> optimizing it. So even if you try (let x = ref 10 in Obj.tag (Obj.repr
> x)) you'll see the same tag, while (as Alain said) Ocaml can actually
> avoid heap allocation in similar expressions where your reference is
> clearly local.

Yes. I understood that looking at the -dlambda compiler option. I didn't
quite grasp the scope of the original question at the time I answered.
 
> The lambda code remains the best option:
> 
> $ cat test.ml
> let () = let x = ref 10 in Printf.printf "%d\n" (Obj.tag (Obj.repr !x))
> let () = let x = ref 10 in Printf.printf "%d\n" (Obj.tag (Obj.repr x))
> 
> $ ocamlc -dlambda test.ml
> (setglobal Test!
>   (let
>     (match/61
>        (let (x/58 10)
>          (apply (field 1 (global Printf!)) "%d\n" (caml_obj_tag (id x/58))))
>      match/60
>        (let (x/59 (makemutable 0 10))
>          (apply (field 1 (global Printf!)) "%d\n" (caml_obj_tag (id x/59)))))
>     (makeblock 0)))
> 
> Cheers,
> -- 
> David

So it's the 'makemutable' that makes all the difference.

By the way, is it possible to compile 'dlambda' code to native code directly?
It would be a nice compilation target.

-- 
     Guillaume Yziquel